Synchro systems

ABSTRACT

A synchro torque receiver has its rotor wound with an additional winding at right-angles to its existing, continuously energized rotor-winding. The three-phase stator-windings of the receiver are energized conventionally from the corresponding windings of a synchro torque transmitter. A control unit detects signals induced in the additional rotor-winding of the receiver and energizes this to create torque aiding the normal reactive torque that drives the receiver-rotor to maintain rotational correspondence between the receiver- and transmitter-rotors. The induced signal is sampled alternately with energization of the additional winding, at a frequency lower than, or the same as, that of the receiver- and transmitter-energization. Alternatively detection and energization of the additional winding can take place concurrently using phase or frequency as the discriminant for detection of the induced signal. Large current-flow into the normal rotor-winding of the receiver, accompanying large deviation from correspondence, is detected to energize the additional winding independently of the induced signal.

United States Patent [72] Inventors Dennis William Dudley Tewkesbury;

Keith Russell Oakey, Cheltenham; George Martin Warner, Cheltenham; John Clement Wright, Stevenage, all of England [21] Appl. No. 40,282 [22] Filed May 25, 1970 [45] Patented Dec. 7, 1971 [73] Assignee Smiths Industries Limited London, England [32] Priority May 23, 1969 [33] Great Britain [3 1 26,468/69 [54] SYNCHRO SYSTEMS 17 Claims, 5 Drawing Figs.

[52] U.S. Cl 318/691, 7 3 18/654 [5|] lnt.Cl ..G05b 11/12 [50] Field 01 Search 318/654, 655, 661, 69]

[56] References Cited UNITED STATES PATENTS 3,381,191 4/1968 Angus 318/654X 3,474,3l2 10/1969 Davis 3,495,146 2/1970 Davis ABSTRACT: A synchro torque receiver has its rotor wound with an additional winding at right-angles to its existing, continuously energized rotor-winding. The three-phase statorwindings of the receiver are energized conventionally from the corresponding windings of a synchro torque transmitter. A control unit detects signals induced in the additional rotorwinding of the receiver and energizes this to create torque aiding the normal reactive torque that drives the receiver-rotor to maintain rotational correspondence between the receiverand transmitter-rotors. The induced signal is sampled alternately with energization of the additional winding, at a frequency lower than, or the same as, that of the receiverand transmitter-energization. Alternatively detection and energization of the additional winding can take place concurrently using phase or frequency as the discriminant for detection of the induced signal. Large current-flow into the normal rotor-winding of the receiver, accompanying large deviation from correspondence, is detected to energize the additional winding independently of the induced signal.

.PATENTEDUEC nan 352625 sum 1 OF 4 aw. pumsv, m; OAKEY G. M. WARNER, J.C,WRIGHT PATENTED DEC 7197! SHEET 2 BF 4 %N rWN vL h QN H H mkiw H JQTQ W RN mm N on Q w. DUDLEY, 1m. OAKEY aim. WARNER J c. WRIGHT by m 2% MM PATENTEDUECTISH 6 $626,266

sumanra aw. DUDLEY, K.R.0A KEY 6M. WARNER, ac wman'r SYNCIIRO SYSTEMS This invention relates to synchro systems.

The invention is particularly concerned with synchro systems of the kind in which plural-phase electrical windings of a synchro device are arranged to be energized electrically in accordance with the value of a variable quantity, and a singlephase electrical winding of thesynchro device is arranged to be energized electrically to establish, by interaction with the energized plural-phase windings, reactive torque that acts to maintain the rotational position of a rotor of the synchro device in predetermined correspondence with. the value of said variable quantity.

Synchro systems of the kind specif ed in the preceding paragraph (hereinafter referred to as -of the kind specified") are commonly used in instrument systems where it is necessary to convey information as to the value of a variable quantity between two spaced stations, as for example where an indication provided by an instrument at one station is to be repeated at the other. These systems commonly include a synchro torque transmitter and a synchro torque receiver. both of these devices having a stator that carries a set of three-phase electrical windings and a rotor that carries a single-phase electrical winding. The rotor-windings of the transmitter and receiver are energized with alternating-current from a common source, and the signals induced in the stator-windings of.

each individual synchro device from. its rotor-winding are dependent upon the rotational position of its rotor. The two sets of stator-windings are coupled together with a star-interconnection, and the signals induced in the stator-windings of the torque transmitter as applied to energize the stator-windings of the torque receiver, are opposed by the signals that within the receiver are induced in the stator-windings from the rotorwinding. The signals from the transmitter are just balanced out in the receiver, only if the rotational position of the receiverrotor has a predetermined correspondence with the rotational.

position of the transmitter-rotor. Any deviation from such correspondence results in current-flow between the interconnected sets of stator-windings and this in the torque receiver gives rise to reactive torque between the stator.- and rotorwindings. This torque acts to rotate the rotor of the. receiver to restore correspondence. 7

One practical disadvantage of synchro systemsof the kind specified is that the magnitude of the reactive torque depends upon the extend of the deviation from correspondence, and for small angles of deviation, may be so small as to impair correct operation of the system. It is an object of the present invention to overcome this disadvantage.

According to the present invention there is provided a synchro system of the kind specified wherein .the synchro;

device has an additional electrical winding that is carried with, and at an angle to, the said single-phase winding, and control means is arranged to respond to an electric signal induced in the additional winding from the plural-phase windings, to apply a further electric signal to the synchro device such as to create additional torque aiding said reactive torque The control means may be arranged to sample intermittently the signal induced in the additional winding and to apply the said further signal to this winding only during the intervals between the sampling. Alternatively it may be arranged that the said further signal is supplied to-energize the addi- I tional winding in phase quadrature with the signal induced in.

the additional winding; detection .of the induced signalin the additional winding may in these circumstances be carried out (for example using a phase-conscious demodulator) concur.

rently with energization thereof.

A synchro system in accordance with the present invention will now be described, by way of example, with reference to.

FIG. 4 is a schematic representation of a form of control unit that may beused in accordance with a modification of the system of FIG. 1; and

FIG. 5 serves to illustrate a modification of the control unit of FIG. 2.

Referring to FIG. 1, a rotatable input shaft 1 of the system is coupled directly to the rotor 2 of a synchro torque transmitter TX. The rotor 2 carries a single-phase electrical winding 3 that is energized with altemating electric current supplied from a source 4 to provide magnetic flux linking the winding 3 to a three-phase electrical winding arrangement carried by the stator 5 of the transmitter TX. The -stator-winding arrangement consists ofthree star-connected windings ,6 to8 that are sym' metrically disposed .with respect to one another about the rotor 2, and thesewindings 6 to 8 are connected via individual lines 9 to 1] to three star-connected electrical windings 12 to 14 carriedby the stator 15 of a synchro torque receiver TR. The windingsl-Z to 14 are symmetrically disposed with respect to one another about the rotor, I6 of the receiver TR, and the rotor 16, which is coupled to an output shaft 17 of the system,

carries a single-phase electrical winding 18. The winding 18, like the rotor-winding 3, is energized with alternating current from the source 4 to provide magneticflux linking the rotorwinding ,18 to the set of stator-windings 12 to 14.

To the extent that thesynchro system of FIG. 1 has so far been described, it is of conventional form, the signals induced in the set of windings lZ to 14 from the energizedrotor-winding 18, balancing out those inducedinthe set of windings 6 to 8 from the energizedrotor-winding 3 only in the event that there is a predetermined correspondence between the rotational position of the input shaft 1 with respect to the stator 5 and the-rotational positionof the output shaft 17 with respect to the stator 15. Any unbalance between the induced voltages, arisingfrom divergence from the condition of correspondence, causes current-flow in the interconnected sets of windings 6 to Sand 12 to 14, and thus results in the establishment within the transmitter. TX and the receiver TRof magnetic flux reactingwith the energized rotor-windings 3 and IS. The reactive torques thatare-consequently exerted on the rotors 2 and 16in these circumstances are such as to tend to rotate .both shafts I and 17 into correspondence .with one another. In accordance with normal practice, however, there is in the present instance sufficient restraint imposed on the input shaft 1 to preclude rotation of the rotor 2 under the reactive torque, and it isrotation of the shaft 17 under the reactive torquezexerted on; the rotor 16 that is solely effective to restore the rotational.correspondence between the shafts I and 17.

The shaft I in the present example is coupled to a transducer device 19 and. it is the positive nature of the drive from this that precludes rotation of the rotor 2 under reactive torque..The transducer 19 is responsive to a variable (such as, for example, airpressure) to maintain the rotational position of the input shaft 1 in accordance with the value of the variable. The output shaft 17, on theother hand, drives a digital indicator 20 and an index 21 that serve respectively to provide digital and analoguerepresentations of the value of the variable in accordance withthe rotational position of the shaft 17. As the value of the .variable changes so the rotational position of the shaftlchanges, and this accordingly results in a reactive torque ontlte rotor 16 to rotate the shaft 17 and maintain the digital and analogue representations'in' true accord with the measured value.

A synchro system having the generally conventional form so fardescribed, suffers from the disadvantage that the reactive torquedeveloped to correctfor divergence. from the desired rotational correspondence. is very dependent upon the magnitude of angular error involved. The smaller the angular. error. the smaller the torquedeveloped, and in practice the reactive torque developed on the rotorof the synchro .torque'receiver in response to small change in rotational position of the input shaft of the synchro torque transmitter may even be insufficient to overcome the frictional loading on the receiver=rotor.

The accuracy of the conventional torque-synchro system in maintaining the desired rotational correspondence is therefore in normal circumstances, quite low; as a practical example the system may maintain correspondence to an accuracy of one degree only. Significant improvement in this accuracy is obtained in the present case, however, by the application to the rotor 16 of the torque receiver TR of additional torque that is derived using an additional single-phase electrical winding 22 carried by the rotor 16.

The winding 22 is wound on the rotor 16 at right angles to the winding 18 so that the voltage induced in the winding 22 is zero when rotational correspondence exists between the shafts l and 17. In the event of any divergence of the shaft 17 from rotational correspondence with the shaft 1 there is induced in the winding 22 an alternating voltage having an amplitude and phasing dependent upon the extent and sense of the angular error involved. The winding 22 is connected to a control unit 23 that samples the induced voltage and, if this exceeds a predetermined threshold, then energizes the winding 22 transitorily to exert torque on the rotor 16 that aids the normal reactive torque in restoring rotational correspondence of the shaft 17 with the shaft 1. The successive steps of sampling the induced voltage and energizing the winding 22 take place in a recurring cycle so that the appropriate additional torque required to bring about and maintain rotational correspondence is applied to the rotor 16 intermittently.

Details of construction of the control unit 23 are shown in FIG. 2, and will now be described.

Referring to FIG. 2, operation of the control unit 23 is governed by an astable trigger circuit 24 that has a periodic time of 40 milliseconds. The output waveform of the circuit 24 is supplied to a monostable trigger circuit 25 and also to a gate 26 that receives via a resistor 27 the signal induced in the winding 22. The gate 26 is open only throughout a first half of each cycle of the output waveform of the trigger circuit 24 and during this half-cycle passes the induced signal to a demodulator-gate 28.

The demodulator-gate 28 is controlled via a buffer amplifier 29 by the alternating-current energization signal supplied from the source 4. The frequency of the energization signal in this case is 400 Hz., and while the gate 26 is open the gate 28 passes current during alternate half-cycles of the energization signal to charge a capacitor 30. The current passed by the gate 28 is dependent in magnitude and sense upon the amplitude and the phasing with respect to the energization signal, of the sampled signal from the winding 22. The voltage developed across the capacitor 30 accordingly provides a measure, in both magnitude and sense, of the angular deviation of the shaft 17 from correspondence with the input shaft 1. This voltage is sampled by a gate 31 during each second half-cycle of the output waveform of the circuit 24.

The gate 31 is controlled by the output waveform of monostable trigger circuit 25, to sample the voltage derived across the capacitor 30 only while the circuit 25 is in its transitory state. The circuit 25 is triggered into the transitory state by the commencement of the second half-cycle of the output waveform of the trigger circuit 24, the duration of this transitory state being milliseconds. The voltage sampled by the gate 31 during this period is supplied from the gate 31 to a modulator-gate 32 that is controlled from the circuit 29 to open during alternate half-cycles of the alternating-current energization signal and pass current in accordance with the magnitude of the voltage-sample. The altemating-current signal derived in this way by the gate 32 is passed to a highgain amplifier 33 and is either in-phase or antiphase with the energization signal according to the sense of the voltage-sample. The amplitude of the signal passed to the amplifier 33 is dependent upon the magnitude of the voltage across the capacitor 30, so the amplifier 33 is supplied from the gate 32 with a pulse of alternating current having an amplitude and phasing dependent upon the magnitude and sense of the angular deviation of the shaft 17 from correspondence with the shaft 1. The amplitude of this pulse exceeds the inputthreshold of the amplifier 33, and thereby causes application of a pulse of alternating current to a power-output stage 34, whenever the deviation exceeds a predetermined small value (for example 7 minutes of arc). The alternating-current pulse supplied in this way from the amplifier 33 is independent in amplitude, but not in phasing, of the deviation, and is supplied from the stage 34 to the winding 22 via a resistor 35.

The power-output stage 34 has a low output impedance to the winding 22 but a high input impedance from it, and the pulse applied to this winding produces a transitory reactive torque on the rotor 16. This torque, being dependent in sense upon whether the applied pulse is in-phase or antiphase with the alternating-current energization signal, acts upon the rotor 16 to reduce the extent of the deviation.

The monostable trigger circuit 25 returns to its stable state, and therefore closes the gate 31, some 10 milliseconds before recommencement of the first half-cycle of the output waveform of the trigger circuit 24. There is therefore, an interval between any alternating-current pulse applied to the winding 22 and the resampling of the induced-signal in this winding. This interval, which enables the system to settle following the applied reactive torque, may in certain applications be found unnecessary and in these circumstances control of the gate 31 may be taken directly from the trigger circuit 24, the monostable circuit 25 being omitted.

The sampling of the induced-signal in the winding 22 is repeated alternately with the application to this winding of pulses of alternating current, through successive cycles of the output waveform of the trigger circuit 24. The application of these pulses from the control unit 23 ceases only when the shaft 17 has been brought so closely into exact correspondence with the shaft 1 that the induced signal in the winding 22 is insufficient to result in the emission of a pulse from the amplifier 33.

With the control unit illustrated with reference to FIG. 2, each step in the recurrent process of monitoring and energizing the additional rotor-winding 22 has a duration significantly longer than the periodic time of the alternating current used for energization. This is not essential, and where the transients encountered with the system are of a significantly higher frequency than the energiiation signal, then it is possible to use one half-cycle of the energization signal for monitoring and the other for energizing the winding 22. The circuit diagram of a form of control unit 23 that may be used in these circumstances is shown in FIG. 3.

Referring to FIG. 3, the signal from the winding 22 is supplied via a limiter-amplifier 36 to a demodulator-gate formed by a transistor 37. The transistor 37, which is connected in the common-emitter circuit configuration and in shunt with a storage capacitor 38, is controlled to conduct during each positive-going half-cycle of the energization signal supplied by the source 4. During the negative-going half-cycle the transistor 37 is nonconductive and so the charge of the capacitor 38 at this time changes in accordance with the magnitude and phasing of the signal induced in the winding 22.

The state of charge of the capacitor 38 is sampled during each positive-going half-cycle of the energization signal using a modulatorgate formed by a transistor 39. The transistor 39, which like the transistor 37 is connected in the commonemitter circuit configuration and in shunt with the capacitor 38, is controlled to conduct during each negative-going halfcycle of the energization signal, and so is is only during each positive-going half-cycle that a sample-signal is passed on to a transistor amplifier 40. The sample-signal amplified by the amplifier 40 is applied via a clamping-gate, formed by a transistor 41, to an output amplifier 42 that is connected to the winding 22 via a resistor 43. The transistor 41 is shuntconnected in the common-emitter circuit configuration and is controlled to conduct only during each negative-going halfcycle of the energization signal. The input-signal to the amplifier 42 is in this way clamped to zero throughout the negativegoing half-cycles of the energization signal, (that is to say, during those intervals for which sampling of the signal in the winding 22 takes place), and it is only during the positivegoing halfncycles that current of magnitude and sense dependent upon the charge accumulated on the capacitor 38 is supplied to the winding 22 from the amplifier 42.

With the synchro system shown in FIG. 1 and incorporating either of the forms of control unit 23 described with reference to FIGS. 2 and 3, the generation of the reactive torque necessary to correct the rotational position of the shaft 17 is achieved by successive steps of monitoring and energizing the winding 22 of the torque receiver TR. These two functions may nonetheless be carried out concurrently, and a form of control unit 23 that can be used for this, is shown in FIG. 4.

Referring to FIG. 4, the signal induced in the-winding 22 is in this case supplied via an amplifier 44 to a phase-conscious demodulator45 so as to derive a voltage having a magnitude and sense dependent upon the amplitude and phasing of the induced signal. The phasing of the induced-signal is compared in the demodulator 45 with that of the altemating-current energization signal supplied by the source 4, and the resultant voltage signal is applied via a filtering or shaping network' (for example a phase advancing network) 46 to a modulator 47 to modulate an altemating-current signal supplied from a phasechanger 48. The phase-changer 48 receives the energization signal supplied by the unit 4 and effects a phase-change of 90, so the modulated signal supplied by the modulator 47 .is in phase quadrature with the energization signal.

The modulated signal derived by, the modulator 47 is supplied via a power amplifier49 to energize the winding 22 and thereby establish reactive torque in the appropriate sense to reduce deviation from angular correspondence between the shafts l and 17. This signal applied to the winding 22, being in phase quadrature with the energization signal supplied to the winding 18, establishes the conjunction with the signal in the winding 18 a small rotating magnetic field that (by inductionmotor effect) rotates the rotor 16 until correspondence is restored. The signal applied to the winding 22 is also in phase quadrature with the signal induced in the winding 22 from the windings 12 to 14, and this fact enables the energization and monitoring of the winding to be carried out concurrently; the phase-conscious demodulator 45 rejects the quadrature-phase signal and responds only to the in-phase or antiphase inducedsignal in the winding 22. I

With the arrangement described above with reference to FIG. 3, using concurrent monitoring and energization of the winding 22, discrimination between inducedand energiza' tion-signals is based on a difference of phasing. This discrimination may alternatively be based on frequency. For example, it may be arranged that the controlled energization of I v the winding 22 is carried out with a signal-frequency different from the basic frequency used for energization of the rotorwindings 3 and 18, the control unit connected to the winding 22 in these circumstances responding to the induced signal of this latter, basic frequency but not to any signal of the frequency used for energization of the winding 22. As a preferred alternative however, a signal-frequency, say of higher value than that used for energization of the rotorwindings 3 and 18, is injected into the energization circuit of the rotor-winding 3, and it can then be arranged that the controlled energization of thewinding 22 is made using the basic energization signal as before, the control unit in this case discriminating against the basic energization-frequency to respond to only those components of the signal induced inthe winding 22 that are of the injected signal-frequency.

It may be found in certain circumstances that the shaft 17 of the synchro torque receiver TR in the system of FIG. 1 tends to adopt a rotational position that is I80 displaced from the position of true correspondence with the shaft 1. Measures to avoid this may be readily incorporated in the control unit 23 and a specific modification that may be adopted in the circuit of FIG. 2 in this respect, is shown in FIG. 5.

Referring to FIG. 5,-the arrangement of FIG. 2 is modified to include a resistor 50 in series with the common, earth connection to the two windings l8 and 22, and to connect the junction between this resistor 50 and the windings l8 and 22 via two oppositely-poled and shunt-connected diodes 51 and 52, to the junction of the resistor 27 with the input-connection to the gate 26. The diodes 51 and 52 are normally nonconductive so as to play no part in the operation of the circuit, but if the shaft 17 tends to adopt a rotational position that is some l displaced from the position of true correspondence then the voltage developed across the resistor 50 rises to a value that causes one or the other of them to conduct. This rise in voltage results from the fact that the current taken by the winding 18 increases substantially (for example, from to 600 milliamperes) when there is substantial deviation from correspondence.

Conduction of either diode-50 and 51 biases the input-connection of the gate 26 to cause a train of pulses of alternating current to be applied to the winding 22. This biasing of the gate 26, and therefore the emission of the train of alternatingcurrent pulses to the winding 22, continues until the pulses have returned the shaft 17 towards true correspondence and the diodes 50 and 51 are both once again nonconductive. The diodes 50 and 51 become nonconductive as the current taken by the winding 18 returns, with return of the shaft 17, to a more normal value (for example 300 milliamperes), and this removes the overriding bias from the input-connection to the gate 26.

The control unit 23 may, as a general principle, be shared between two or more synchro systems, the torque receivers of the different systems being, for example, connected to the unit 23 in recurring sequence.

We claim:

1. A synchro system comprising:

a synchro device having a stator-part,

a rotor-part mounted for rotationrelative to the stator-part,

plural-phase electrical windings carried by one of the two two single-phase electrical windings carried by the other part and coupled inductively to the plural-phase windings,

the two single-phase windings being disposed at an angle to one another; means responsive to an input variable to energize the pluralphase windings in accordance with the value of the variable;

means for energizing a first of the single-phase windings to establish, by interaction with the energized plural-phase windings, reactive torque acting to maintain the rotational position of the rotor-.part in predetermined correspondence with the value of said variable;

and control means responsive to an electric signal induced in the second single-phase winding from the energized plural-phase-windings to apply a further electric signal to the said second winding to generate additional torque aiding said reactive torque,

said control means comprising signal-applying means operable to apply to said second winding as said further signal a signal having a predetermined characteristic, signal-discriminating means for discriminating against any signal in said second winding having said characteristic to derive from;said second winding a control signal depen- '.dent,,on the induced signal and substantially independent of the said further signal, and means for applying the said control signal to operate the signal-applying means to apply the further signal to the second winding.

2. A synchro system according to claim I wherein said control means includes means responsive to the condition in which there is substantial deviation from said predetermined correspondence to energize said second windingv independently of the magnitude of said induced signal.

3. A synchro system according to claim 1 ,wherein said signal-discriminating means includes means for intermittently sampling the signal induced in said second ,winding to discriminate against any signal absent from said winding during the periods of sampling, and wherein said signal-applying means is means operable to apply said further signal to the said second winding only during the intervals between sampling.

4. A synchro system according to claim 1 wherein said control means comprises first means coupled to said second winding to detect the appearance of a first electric signal-component therein, and second means responsive to the detected signal-component to energize said second winding with a second electric signal-component different from said first signal component, said first means including means to discriminate against said second signal-component and thereby select only said first signal-component for detection.

5. A synchro system according to claim 4 wherein said first and second signal-components differ from one another in their phasing.

6. A synchro system according to claim 1 wherein said control means includes means for deriving said further signal as a signal in phase quadrature with said signal induced in the second winding, and means for applying said phase-quadrature signal to energize said second winding.

7. A synchro system according to claim 1 wherein said second winding is at right-angles to said first winding.

8. A synchro system according to claim 1 wherein said first and second windings are both carried by the rotor-part.

9. A synchro system comprising:

a synchro torque receiver comprising a stator, three-phase electrical windings carried by the stator, a rotor mounted for rotation relative to the stator, two electrical windings carried by the rotor,

said two rotor-windings being disposed at right-angles to one another on said rotor;

transmitter means responsive to an input variable to derive cyclically-varying electric signals that are dependent in amplitude upon the value of said variable;

means for supplying the signals derived by said transmitter means to energize the said stator-windings of said torque receiver;

means coupled to a first of the rotor-windings for energizing said first rotor-winding with a cyclically varying electric signal to establish, by interaction with the energized stator-windings, reactive torque acting to maintain the rotational position of the rotor in predetermined correspondence with the value of said variable;

detecting means coupled to the second rotor-winding to detect an electric signal induced in the second rotor-winding from the stator-windings,

said detecting means including means operative to discriminate against any signal having a predetermined characteristic in the detection of the signal induced in said second rotor-winding;

and supply means responsive to detection of the induced signal by the said detecting means to energize the said second rotor-winding with a further cyclically varying electric signal to aid said reactive torque in maintaining said predetermined correspondence,

the said further electric signal being a cyclically varying electric signal having said predetermined further characteristic whereby the signal detection performed by said detecting means is substantially independent of said further signal.

10. A synchro system according to claim 9 wherein said transmitter means includes a synchro torque transmitter having a stator that carries three-phase electrical windings and a rotor that carries an electrical winding, means connecting the stator-windings of the torque transmitter to the statorwindings of the torque receiver, and means energizing the rotor-winding of the torque transmitter with the said cyclically-varying energization signal supplied to energize the said first rotor-winding.

ll. A synchro system according to claim 9 wherein said detecting means includes sampling means for sampling the signal induced in the said second rotor-winding only during a regularly-recurrent period, and said supply means includes means for supplying an electric signal dependent on the sampled signal to energize the second rotor-winding only in the intervals between the periods of sampling.

12. A synchro system according to claim 11 wherein the said sampling means is means to sample the signal induced in the said second rotor-winding during a recurrent period that is substantially longer than the period of the cyclically varying signal supplied to energize said first rotor-winding.

13. A synchro system according to claim 11 wherein the said sampling means is means to sample the signal induced in the said second rotor-winding during a first part of the cycle of the said cyclically-varying signal supplied to energize the said first rotor-winding, and said supply means is means for energizing the second rotor-winding during a second part of the cycle.

14. A synchro system according to claim 9 including means responsive to the condition in which there is substantial deviation from said predetermined correspondence to effect energization of said second rotor-winding independently of the amplitude of said induced signal.

15. A synchro system according to claim 14 wherein said means responsive to the condition in which there is substantial deviation from correspondence is means responsive the the condition in which electric current-flow in said first rotorwinding exceeds a predetermined threshold level.

16. A synchro system comprising:

a synchro torque receiver comprising a stator, three-phase electrical windings carried by the stator, a rotor mounted for rotation relative to the stator, two electrical windings carried by the rotor,

said two rotor-windings being disposed at right-angles to one another on said rotor;

transmitter means responsive to an input variable to derive cyclically-varying electric signals that are dependent in amplitude upon the value of said variable;

means for supplying the signals derived by said transmitter means to energize the said stator-windings of said torque receiver;

means coupled to a first of the rotor-windings for energizing said first rotor-winding with a cyclically varying electric signal to establish, by interaction with the energized stator-windings, reactive torque acting to maintain the rotational position of the rotor in predetermined correspondence with the value of said variable;

and control means coupled to the second rotor-winding to respond to an electric signal induced in the second rotor winding from the stator-windings,

said control means responding to the induced signal to energize the second rotor-winding electrically to aid said reactive torque in maintaining said predetermined correspondence and comprising sampling means for sampling the signal induced in the said second rotor-winding during a regularly-recurrent period,

a capacitor,

means for charging the capacitor in accordance with the amplitude of the sampled signal,

and means to energize the second rotor-winding in accordance with the state of charge of the capacitor in the intervals between the periods of sampling.

17. A synchro system comprising:

a synchro device having a, stator-part,

a rotor-part mounted for rotation relative to the stator-part,

plural-phase electrical windings carried by one of the two parts,

two single-phase electrical windings carried by the other part and coupled inductively to the plural-phase windings,

the two single-phase windings being disposed at an angle to one another;

means responsive to an input variable to energize the pluralphase windings in accordance with the value of the variable;

torque and reduce said deviation.

said control means comprising detector means to detect the condition in which electric current flow in said first winding exceeds a predetermined threshold value,

and means to energize the second single-phase winding electrically in response to detection of said condition by said detector means. 

1. A synchro system comprising: a synchro device having a stator-part, a rotor-part mounted for rotation relative to the stator-part, plural-phase electrical windings carried by one of the two parts, two single-phase electrical windings carried by the other part and coupled inductively to the plural-phase windings, the two single-phase windings being disposed at an angle to one another; means responsive to an input variable to energize the pluralphase windings in accordance with the value of the variable; means for energizing a first of the single-phase windings to establish, by interaction with the energized plural-phase windings, reactive torque acting to maintain the rotational position of the rotor-part in predetermined correspondence with the value of said variable; and control means responsive to an electric signal induced in the second single-phase winding from the energized plural-phase windings to apply a further electric signal to the said second winding to generate additional torque aiding said reactive torque, said control means comprising signal-applying means operable to apply to said second winding as said further signal a signal having a predetermined characteristic, signal-discriminating means for discriminating against any signal in said second winding having said characteristic to derive from said second winding a control signal dependent on the induced signal and substantially independent of the said further signal, and means for applying the said control signal to operate the signal-applying means to apply the further signal to the second winding.
 2. A synchro system according to claim 1 wherein said control means includes means responsive to the condition in which there is substantial deviation from said predetermined correspondence to energize said second winding independently of the magnitude of said induced signal.
 3. A synchro system according to claim 1 wherein said signal-discriminating means includes means for intermittently sampling the signal induced in said second winding to discriminate against any signal absent from said winding during the periods of sampling, and wherein said signal-applying means is means operable to apply said further signal to the said second winding only during the intervals between sampling.
 4. A synchro system according to claim 1 wherein said control means comprises first means coupled to said second winding to detect the appearance of a first electric signal-component therein, and second means responsive to the detected signal-component to energize said second winding with a second electric signal-component different from said first signal component, said first means including means to discriminate against said second signal-component and thereby select only said first signal-component for detection.
 5. A synchro syStem according to claim 4 wherein said first and second signal-components differ from one another in their phasing.
 6. A synchro system according to claim 1 wherein said control means includes means for deriving said further signal as a signal in phase quadrature with said signal induced in the second winding, and means for applying said phase-quadrature signal to energize said second winding.
 7. A synchro system according to claim 1 wherein said second winding is at right-angles to said first winding.
 8. A synchro system according to claim 1 wherein said first and second windings are both carried by the rotor-part.
 9. A synchro system comprising: a synchro torque receiver comprising a stator, three-phase electrical windings carried by the stator, a rotor mounted for rotation relative to the stator, two electrical windings carried by the rotor, said two rotor-windings being disposed at right-angles to one another on said rotor; transmitter means responsive to an input variable to derive cyclically-varying electric signals that are dependent in amplitude upon the value of said variable; means for supplying the signals derived by said transmitter means to energize the said stator-windings of said torque receiver; means coupled to a first of the rotor-windings for energizing said first rotor-winding with a cyclically varying electric signal to establish, by interaction with the energized stator-windings, reactive torque acting to maintain the rotational position of the rotor in predetermined correspondence with the value of said variable; detecting means coupled to the second rotor-winding to detect an electric signal induced in the second rotor-winding from the stator-windings, said detecting means including means operative to discriminate against any signal having a predetermined characteristic in the detection of the signal induced in said second rotor-winding; and supply means responsive to detection of the induced signal by the said detecting means to energize the said second rotor-winding with a further cyclically varying electric signal to aid said reactive torque in maintaining said predetermined correspondence, the said further electric signal being a cyclically varying electric signal having said predetermined further characteristic whereby the signal detection performed by said detecting means is substantially independent of said further signal.
 10. A synchro system according to claim 9 wherein said transmitter means includes a synchro torque transmitter having a stator that carries three-phase electrical windings and a rotor that carries an electrical winding, means connecting the stator-windings of the torque transmitter to the stator-windings of the torque receiver, and means energizing the rotor-winding of the torque transmitter with the said cyclically-varying energization signal supplied to energize the said first rotor-winding.
 11. A synchro system according to claim 9 wherein said detecting means includes sampling means for sampling the signal induced in the said second rotor-winding only during a regularly-recurrent period, and said supply means includes means for supplying an electric signal dependent on the sampled signal to energize the second rotor-winding only in the intervals between the periods of sampling.
 12. A synchro system according to claim 11 wherein the said sampling means is means to sample the signal induced in the said second rotor-winding during a recurrent period that is substantially longer than the period of the cyclically varying signal supplied to energize said first rotor-winding.
 13. A synchro system according to claim 11 wherein the said sampling means is means to sample the signal induced in the said second rotor-winding during a first part of the cycle of the said cyclically-varying signal supplied to energize the said first rotor-winding, and said supply means is means for energizing the second rotor-winding during a second part of the cycle.
 14. A syNchro system according to claim 9 including means responsive to the condition in which there is substantial deviation from said predetermined correspondence to effect energization of said second rotor-winding independently of the amplitude of said induced signal.
 15. A synchro system according to claim 14 wherein said means responsive to the condition in which there is substantial deviation from correspondence is means responsive to the condition in which electric current-flow in said first rotor-winding exceeds a predetermined threshold level.
 16. A synchro system comprising: a synchro torque receiver comprising a stator, three-phase electrical windings carried by the stator, a rotor mounted for rotation relative to the stator, two electrical windings carried by the rotor, said two rotor-windings being disposed at right-angles to one another on said rotor; transmitter means responsive to an input variable to derive cyclically-varying electric signals that are dependent in amplitude upon the value of said variable; means for supplying the signals derived by said transmitter means to energize the said stator-windings of said torque receiver; means coupled to a first of the rotor-windings for energizing said first rotor-winding with a cyclically varying electric signal to establish, by interaction with the energized stator-windings, reactive torque acting to maintain the rotational position of the rotor in predetermined correspondence with the value of said variable; and control means coupled to the second rotor-winding to respond to an electric signal induced in the second rotor-winding from the stator-windings, said control means responding to the induced signal to energize the second rotor-winding electrically to aid said reactive torque in maintaining said predetermined correspondence and comprising sampling means for sampling the signal induced in the said second rotor-winding during a regularly-recurrent period, a capacitor, means for charging the capacitor in accordance with the amplitude of the sampled signal, and means to energize the second rotor-winding in accordance with the state of charge of the capacitor in the intervals between the periods of sampling.
 17. A synchro system comprising: a synchro device having a stator-part, a rotor-part mounted for rotation relative to the stator-part, plural-phase electrical windings carried by one of the two parts, two single-phase electrical windings carried by the other part and coupled inductively to the plural-phase windings, the two single-phase windings being disposed at an angle to one another; means responsive to an input variable to energize the plural-phase windings in accordance with the value of the variable; means for energizing a first of the single-phase windings to establish, by interaction with the energized plural-phase windings, reactive torque acting to maintain the rotational position of the rotor-part in predetermined correspondence with the value of said variable; and control means responsive to the circumstances in which there is substantial deviation from said predetermined correspondence to generate torque aiding said reactive torque and reduce said deviation, said control means comprising detector means to detect the condition in which electric current flow in said first winding exceeds a predetermined threshold value, and means to energize the second single-phase winding electrically in response to detection of said condition by said detector means. 